PROJECT SUMMARY/ABSTRACT Sports-related concussion in youth is increasingly being recognized as a public health concern. Decreasing the health burden of concussion requires both (a) decreasing the number of concussions that occur by decreasing dangerous collisions (primary prevention) and (b) ensuring that youth are not playing with concussive symptoms and thereby incurring greater risk (secondary prevention). Rules are a promising means for effecting such changes (i.e, outlawing spearing, requiring that athletes be removed from play if a concussion is suspected), but changing rules is not effective if we do not also shift the culture of youth sport regarding safety. Sport cultures that reinforce values of winning-at-all-costs lead athletes, coaches and parents to disregard the rules that are intended to keep athletes safe. Referees, many of whom are often minors themselves, face challenges enforcing rules in such environments. Changing the culture of sport requires a paradigm shift in how we think about intervention. Prior efforts to decrease the incidence of concussion and decrease the rate of youth playing with concussive symptoms have had limited effectiveness because they did not intervene at a systems-level. Changing sport culture requires intervening with multiple stakeholders (referees, coaches, athletes, parents) and addressing the values that underlie a commitment to following rules related to concussion safety. We propose an intervention called ?One Team? that will utilize safety huddles to shift the culture of youth sport. The huddle was originally developed in football, but it has been adopted by the medical system as a way to engage key stakeholders to improve safety. The One Team intervention will use pre-game safety huddles to bring together coaches, athletes, parents and referees to affirm: (a) values of sportsmanship (i.e, not engaging in dangerous and illegal collisions) and (b) shared responsibility that no athlete play while concussed. Working in partnership with football and soccer coaches, athletes, parents and referees in Seattle WA and Statesboro GA using a community-engaged process, we will refine the One Team intervention and conduct pilot evaluations of implementation and efficacy. We will also work with our national partners (Centers for Disease Control and Prevention, US Soccer and USA Football) throughout this research to ensure that the product we develop will align with their organizational needs, dissemination platforms and channels. At the completion of this research we will have an intervention that will shift the culture of safety in youth sport and that can reach all youth sport stakeholders, including those in low resource communities.